Buddhanet: The Buddha Dharma Education Association provides a vast collection of pdf books for download. Well worth a visit.

Dhammapada: This is the complete translation in 26 chapters by Max Muller, 1881, as part of the Sacred Books of the East project. Another version, translated by Ven. Thanrissano, Bikkhu in .pdf

Diamond Sangha Sutras: these are the sutras and chants translated by Robert Aitken and chanted by the Diamond Sangha. Includes the Heart Sutra, the Sho- Sai Myo- Kichijo- Darani,
Emmei Jikku Kannon Gyo, Hakuin's Song of Zazen as well as many others.

English Translations of Mahayana Sutras: this site has an extensive selection of translated sutras. For example, it has 15 translations of the Heart Sutra. All the major sutras are here. Well worth visiting, a bodhisattva site.

The Essence of Mahayana Practice
Translated by the Chung Tai Translation Committee, May 2008
From the Chinese by The First Patriarch Bodhidharma, 6th Century This is Bodhidharma's teaching on entering the Great Way. More translations by the Chung Tai Translation Committee can be found here.

Fodian.net : This is an excellent source of English translations of Buddhist texts. Many of the sutras are translated by the Buddhist Text Translation Society. A very big site that has all the major sutras here. Recommended.

D.T. Suzuki's Manual of Zen Buddhism, first published in 1935; includes sutras, gathas, dharanis, translations of some of the writings of Chinese Zen masters and more.

The Pali Canon: Tipitaka:
A wide selection, free for downloading. Sayings of the Buddha in 42 Sections: This text was said to be the first official Buddhist literature which was composed for the Chinese by two early Indian missionaries, Kashyapa Matanga and Gobharana. The translators extracted all the passages from different Buddhism scriptures which they brought along for their missionary purposes. It contains a good collection of moral and religious sayings of the Buddha.

Siddham.org: there are a number of sutra translations here. Most are links to other sites.

The Sweet Dew of Chan: Lectures on Buddhist Meditation by Rev. Cheng Kuan. This is a 92-page pdf book supplied by Buddha Dharma Education Association, Inc.

The Gospel of Buddha: this is an electronic version of Paul Carus' 1894 translations of some essential Buddhist teachings.

The Great Prajnaparamita Heart Sutra

chanted daily in almost every Zen temple in the world. Commentary by Grand Master T'an Hsu. Translated into English by Venerable Dharma Master Lok To, Young Men's Buddhist Association, New York
There's quite a good rendering of the Japanese Hannya Shingo with a word-by-word translation here.
The Heart Sutra used by the Diamond Sangha is here.
Translation by E. Conze is here.
The version used by the Western Ch'an Fellowship is here, includes links to other translations.
The English version used by the Dzogchen Center is here, including a RealAudio listening. A number of talks on this fundamental text by Sojun Mel Weitsman of the Berkeley Zen Center. The Shunryu Suzuki translation of the Heart Sutra.
Commentary by the Venerable Master Hsüan Hua, translated by Ron Epstein

Other Specific Sutras

Decline of the Dharma: Not a genuine sutra but a "spurious scripture" from possible 6th Century China. Full title: The Scripture Preached by the Buddha on the Total Extinction of the Dharma. This one includes a useful introduction by Randall L Nadeau Originally published: B.C. Asian Review, Vol. 1, 1987 original source

Diamond Sutra: a new translation by Alex Johnson. This is the complete version online. Western Chan Fellowship has a version here as well as links to other translations.

Mahayana Mahaparinirvana Sutra (Nirvana Sutra) from the introduction page: "This site is the world's first-ever website centred on an exploration and appreciation of the Buddha's final Mahayana scripture, the Mahayana Mahaparinirvana Sutra. This site also provides a complete English translation of the entire sutra (the 36-fascicle "Southern" version)."

Nagarjuna: romanized Tibetan text of Nagarjuna’s Mulamadhyamakakarika together with a literal English translation by Stephen Batchelor

Another complete version, translated by The Buddhist Text Translation Society with commentary by Tripitaka Master Hua is also available. Here as a 454 page .pdf document (not recommended for slow connections) John R McRae's translation is here (172 pages)

Carl Bielefeldt and Lewis Lancaster: T'an Ching (Platform Scripture) Bielefeldt and Lancaster give an excellent overview of state of current (1975) translations and scholarship of the Platform Sutra.
from Philosophy East and West Vol. 25, No. 2, 1975

Morten Schlütter: Transmission and Enlightenment in Chan Buddhism Seen Through the Platform Sūtra (Liuzu tanjing 六祖壇經) This paper discusses developments in the conceptualization of transmission and enlightenment in the Platform Sutra's main line of development, beginning with the earliest version found at Dunhuang leading down to the orthodox version included in the Taishō canon. from Chung-Hwa Buddhist Journal, no. 20, pp. 379～410 (2007)
Autobiography of Hui Neng translated by A.F.Price and Wong Mou-Lam This is an excerpt from the Platform Sutra

Huang Po

Huang Po: (J. Obaku) a small sample of some of the teachings of this great master. He is believed to have died in 850 A.D. and many consider him the founder of the Lin Chi (J. Rinzai) school. The only complete translation is John Blofeld's The Zen Teachings of Huang Po. Read a "gonzo review" here.

Hsi Yun: The Ultimate Reality Transcends What Can Be Expressed in Words "The document, here employed, in abbreviated form, presents the teaching of Hsi Yun, one of the Ch’an (or Zen) masters who lived about 840 A.D. His teaching is reported by P’ei Hsiu, an official and scholar who became a student under Hsi Yun. It gives a more or less sympathetic disclosure of Ch’an philosophy."

Hsu Yun: Master Hsu Yun's Discourses
and Dharma Words
Edited, Translated and Explained by
Lu Kuan Yu [Charles Luk], Including: An Introduction to Empty Cloud by Richard Hunn and Master Hsu Yun's sermons and discourses of December 1952
Hsu Yun (1840-1959) (Empty Cloud) is one of the twentieth century's most venerated Chinese Zen masters.
This free download includes a biography of the master, discources on Ch'an training, and daily lectures during a two-week training session. This 135-page pdf document includes photos of the master.

Lin Chi (J. Rinzai) Irmgard Schloegl's classic translation of The Zen Teachings of Rinzai is available in its entirety here as a .pdf document. Free. (86 pages--be patient)The Record of Linji: translation and commentary by Ruth Fuller Sasaki; a true classic translation loaded with commentaries. Fairly slow loading as it is 521 pages. First published in 1975

Nagarjuna: The Precious Garland Ratnavali of Nagarjuna also known as Nagarjuna's Letter to a Friend, this is advice given by Nagarjuna to the Southern Indian King Surabhidhadra in 123 four-line verses. This version has broken the writing down into 500 verses.